The combined power generation using the gas turbine and steam turbine generates power by driving the gas turbine by high-temperature gas obtained by burring a power generation fuel such as natural gas, and furthermore, it recovers the heat of the exhaust gas of the gas turbine as steam so as to drive the steam turbine and thereby generate power; therefore, it is more efficient than the boiler power generation, which recovers the heat of the gas obtained by burning the power generation fuel as steam and driving the steam turbine.
As a method for executing the combined power generation using carbon based fuel such as coal as fuel, there is a gasification power generation system. In this system, the carbon based fuel in the gasifier is gasified using an oxidizing agent such as oxygen and the obtained syngas is used as fuel.
When executing the combined power generation using natural gas containing methane a main component as the power generation fuel, the heating value of the natural gas is almost constant, so that the air quantity for the natural gas fed to a gas turbine combustor is calculated using a preset heating value of the natural gas, thereby the combustion state in the gas turbine combustor can be maintained stably.
By contrast, when using the syngas obtained by gasifying the carbon based fuel as fuel for the gas turbine combustor, the ratio of the oxidizing agent such as oxygen used for gasification to the carbon based fuel needs to be adjusted depending on the operation state of the gasifier, so that the heating value of the syngas obtained in the gasifier is varied.
Further, for the pressure detection pipe in the gasifying process, to prevent the blocking by pulverized coal which is carbon based fuel, it is necessary to always feed a small quantity of purge nitrogen, though the nitrogen feed rate is generally set to a regulated flow rate by a hand valve, so that it is possible that the nitrogen feed rate will increase or decrease with lapse of time and as a result, the heating value of the syngas is varied.
For this reason, when the heating value of the syngas used for controlling the gas turbine is set to a fixed value, the real heating value of the syngas may exceed or fall below the value preset as a heating value of the syngas.
If the setting of the heating value of the syngas is different, the syngas quantity and air quantity that become fuel fed to the gas turbine combustor may become too much or too little and there are possibilities that the nitrogen oxide concentration and carbon monoxide concentration in the exhaust gas of the gas turbine discharged from the gas turbine may increase.
Further, there are possibilities that the flame going-out of the gas turbine combustor or the damage to the gas turbine combustor due to a high-temperature flame may be caused.
Therefore, when the carbon based fuel such as coal is gasified in the gasifier and the syngas is used as fuel of the gas turbine in a combined power generation plant, it is necessary to devise how to suppress the variation in the heating value of the syngas or set the heating value of the syngas used to control the gas turbine combustor to an appropriate value in the gasifying process of converting the carbon based fuel to syngas.
For example, in Japanese Patent Laid-open No. Sho 63 (1988)-20388 (Patent Literature 1), the heating value of the syngas is inferred from the concentration ratio of carbon monoxide to carbon dioxide in the gas sampled in the neighborhood of the gasifier outlet or from the gas temperature in the gas turbine combustor, and then the ratio of the carbon based fuel fed to the gasifier to oxygen is increased or decreased, thereby keeping the heating value of the syngas at the inlet of the gas turbine constant.
In Japanese Patent Laid-open No. 2010-285564 (Patent Literature 2), when the gasifier pressure is reduced, it is decided that the heating value has been reduced, and the carbon based fuel feed rate is increased, and when the gasifier pressure is increased inversely, it is decided that the heating value is increased, thereby the carbon based fuel feed rate is reduced, thus the heating value of the syngas is held fixed.
This method is valid when operating the system under the conditions that the power generation target is fixed and the carbon based fuel feed rate is almost constant. However, when the power generation output need to be changed, the gasifier pressure varies due to a factor other than the variation of the heating value of the syngas, so that a problem arises that the heating value of the syngas cannot be kept constant.
In Japanese Patent Laid-open No. 2004-124851 (Patent Literature 3), from the equipment between the gasifier and the gas turbine or the pipes, the syngas is sampled, and the concentrations of the carbon monoxide and carbon dioxide are measured, and from the ratio of the carbon monoxide and carbon dioxide, the heating value of the syngas is inferred. Further, in the pipe between the gasifier and the gas turbine, the flow rate of the syngas is measured, and the arrival time of the syngas from the gas sampling position to the gas turbine is calculated, and in consideration of this time, the flow rate is set as a heating value of the syngas at the gas turbine inlet. This method is valid when the time required from sampling of the syngas to obtaining of the analytical results is short.